Competing mechanisms of Norrish and Norrish-like reactions in a wide range of systems – From carbonyl compounds to nitrogen oxide donators

Structural Causes of Singlet/triplet Preferences of Norrish Type II Reactions in Carbonyls

“…Unfortunately, there are no reliable experimental benchmarks for the S 1 NTII intrinsic reaction barriers; the ranges extracted from experiment are not sufficiently precise. There are a few previous literature calculations of these S 1 intrinsic reaction barriers, 5,26,54 and these are included in Table S1, where they are compared to calculations with four linear response methods (TD-CAM-B3LYP, TD-M06-2X, TD-B2GP-PLYP and EOM-CCSD) on all carbonyl species in this paper. In short, large energetic discrepancies are seen between calculations using these different methods.…”

“…4,52 Multiconfigurational calculations on prototypical aldehydes Kletskii et al used the multiconfigurational XMCQDPT2 method 53 to characterise Norrish reactions in butanal, pentanal, and but-2-enal. 54 XMCQPDT2 provides a correct representation of wavefunctions with multireference character, and regions with electronic state degeneracies such as conical intersections, both of which are incorrectly modelled with linear response (LR) time-dependent density functional theory (TD-DFT) or equationof-motion coupled cluster (EOM-CC) methods. 55 However, like CASPT2, the XMCQDPT2 method is more computationally demanding than single-reference methods, 56 and the energetics delivered for carbonyl photochemistry can often be worse than those from many DFT functionals.…”

“…An (8,7) active space was used for initial optimisations, matching previous literature. 50,54,71 The four occupied orbitals in the active space are the: γC H σ, C O σ, C O π, and oxygen lone pair n orbitals; and the virtual orbitals are the: γC H σ * , C O σ * , and C O π * orbitals. This was then expanded to a (10,8) active space by including the other oxygen-centred non-bonding orbital (see Figure S1).…”

“…The MECI structures calculated here with the 6-31+G(d) basis set for butanal, pentanal, pentan-2-one, and but-2-enal were consistent with those calculated by Kletskii et al with the larger 6-311++G(d,p) basis set. 54 Further details are provided in the supporting information.…”

A comparison of methods for theoretical photochemistry: Applications, successes and challenges

“…As the algorithm was shown to give good, smooth results, it was proposed that it can therefore be used in dynamic simulations. Kletskii et al 103 studied the competition between Norrish-type reactions in butanal, pentanal, penta-2-one, buten-2-al, butanimine-1, and nitrosopropane with CASSCF and XMCQDPT2. For each molecule, the possible dissociation and H-transfer pathways were studied in order to determine their relative competition, finding that increasing alkyl chain length had limited effect on the Norrish-I reaction pathway, but decreased the energy of the H-transfer reactions.…”